1. Field of the Invention
The invention relates to semiconductor devices which are obtained by cutting a wafer encapsulated by an encapsulating material layer in such a manner that each of end faces of bumps for an external terminal is exposed, into individual elemental devices, and to a method of the manufacture thereof.
2. Description of the Related Art
Recent trends of downsizing and miniaturization of electronic equipment are accompanied by development of miniaturized and thinner semiconductor devices having a higher quality. Prior semiconductor devices used a semiconductor functional element and a lead frame and, in such a device, areas other than sites required for the device to be mounted on a print substrate were encapsulated to be coated by an encapsulating material for the purpose of protection against an external environment. Accordingly, development for miniaturization of semiconductor devices was mainly directed to lead frames, which act as a substrate, and encapsulating materials. As a result, TSOPS (thin small outline packages) which have a smaller capacity of encapsulating material and are thinned, QFPs (quad flat packages) which cope with a larger number of pins, etc. have been developed.
A semiconductor device having a lead frame and a larger number of pins has a larger package size, and is more difficult to mount since the outer leads have a smaller pitch. Packages of BGA (ball grid array) type and chip size packages (CSPs), which have been recently developed, are semiconductor devices having the above problems solved. By way of example, the chip size packages include those obtained by, for example, encapsulating a wafer provided on a surface with bumps for external terminals for each functional element (chip) in such a manner that each of end faces of bumps is exposed, and subsequently cutting the wafer into individual chips.
For the encapsulation of semiconductor devices, since a transfer molding using an encapsulating material based on an encapsulating resin component is employed, whereby an outer shape of a semiconductor device is formed by filling the encapsulating material into a mold containing the semiconductor device, mold releasability of an encapsulating material has been an important requirement. Also, there have been problems such that if adhesion between an encapsulating material and a semiconductor functional element or a lead frame is poor, debonding is caused between them, and a debonded portion is prone to be subjected to invasion by external water, to thereby accelerate corrosion of wirings and pads on the functional element, or cause package cracks due to gasification of water during reflowing for mounting of a chip. For an encapsulating material, however, mold releasability during molding and adhesion for protection of a semiconductor functional element are conflicting characteristics, and hitherto, an optimum point has been found by balancing the both. Also, selection, as a material for a semiconductor functional element, of a passivation material such as a polyimide coating material has taken place, or development for better adhesion to an encapsulating material by, for example, making the surface of a lead frame uneven, has been carried out. Nevertheless, in semiconductor devices using a prior encapsulating material it is difficult to make adhesion and mold releasability compatible with each other, and no semiconductor devices have been obtained which fully satisfy such objects.
On the other hand, it has been required that an internal stress caused by use of different materials is reduced to thereby enhance reliability of a semiconductor device, and this requirement has been possible to be satisfied by harmonizing coefficients of thermal expansion of a semiconductor functional element or lead frame and an encapsulating material with each other to reduce a thermal stress. By means of this, reliability of a semiconductor device has been improved.
As an encapsulating material for semiconductor devices, a composition based on an epoxy resin and comprising an inorganic filler such as alumina, silica or the like is used. In such a composition, particularly when a silica filler is used, the surface of the filler is treated by a silane coupling agent or the like, or a coupling agent is mixed with a resin during kneading of the resin, in order to improve the wettability of the inorganic filler with the resin.
An elevated temperature (for example, 175.degree. C.) is used during encapsulation using a resin based encapsulating material. Consequently, in the case of encapsulation of a larger silicon wafer, a warp of the wafer occurs in the course of cooling to room temperature after the encapsulation due to a difference between shrinkage factors of the silicon wafer and the encapsulating material. This warp is an obstacle when solder balls are provided on bumps for individual elements (chips) or the wafer is cut into individual elements (chips).
The encapsulating material comprising a resin component and an inorganic filler had a restrained amount of added filler of 70% by weight or less in order to control the warp of wafer due to curing and shrinkage of the encapsulating material after the encapsulation of the wafer by molding as referred to above. This is because in the amount of more than 70% by weight, the encapsulating material has a higher Young's modulus and a larger shrinkage stress, and accordingly, the warp increases. Also, this is because there had been a problem of largely reduced workability in a manufacturing process of semiconductor device after encapsulation when the warp of a wafer is increased.
In a chip size package, adhesion between an encapsulating material and a chip is important, and a slight peeling of the encapsulating material noticeably reduces reliability of the package. The encapsulating material has a thickness of 80 to 120 micrometers, and the chip is protected only by the encapsulating material. Accordingly, the encapsulating material is required to have more toughness and mechanical strength than that in a prior semiconductor device having a lead frame. In addition, small voids in a layer of the encapsulating material largely reduce reliability of the package.
It is thought that the voids are generated for the following reasons. When silica powder of an inorganic filler is directly treated in advance by a coupling agent, the silane coupling agent and hydroxyl groups on the surface of the silica give rise to a hydrolysis reaction, and the silane coupling agent and the silica chemically bond to each other, while the surface of silica is coated with the silane coupling agent. As the silane coupling agents used here, the following are enumerated: silane coupling agents of glycidyl type such as gamma-glycidoxypropyltrimethoxysilane and gamma-glycidoxypropyltriethoxysilane, and silane coupling agents of amine type. The reaction of a silane coupling agent with a hydroxyl group on a surface of silica is a dealcohol reaction as shown below: ##STR1##
The alcohol ROH generated by this reaction is released to the atmosphere during the treatment of the filler, and therefore does not remain on the surface of the filler. However, there is a possibility of retention of unreacted silane coupling agent and, in a semiconductor device to which an encapsulating material is applied, the encapsulating material being filled with the filler treated by the silane coupling agent, or obtained by previously adding the coupling agent to a resin component and then mixing the mixture with the filler, the unreacted silane coupling agent in the encapsulating material causes, due to effects of ambient temperature and humidity, a hydrolysis reaction with absorbed water, as shown below, to thereby result in generation of an alcohol. ##STR2##
The alcohol in the encapsulating material is abruptly vaporized and expanded by rapid increase in temperature during a molding operation for encapsulation, and causes voids and cracks to be generated in a cured encapsulating material. This is also true to the time of mounting, and water further intrudes into the voids or cracks, resulting in failure of operation of semiconductor functional element. Further, electrical conductivity of the encapsulating material is temporarily reduced during the proceeding of the hydrolysis reaction of the coupling agent, and accordingly, when unreacted coupling agent remains even after mounting, short-circuits between wiring may occur due to the hydrolysis.
On the other hand, a fused silica, among silica powders, is used, in some cases, as a filler to be used in an encapsulating composition, because of its smaller contents of metals, and sodium and chloride ions. Since few hydroxyl groups are present on the surface of the fused silica, a dealcohol reaction does not take place between the fused silica filler and a silane coupling agent when the filler is treated by a silane coupling agent as stock solution. Accordingly, there has been a problem of a semiconductor device to which an encapsulating material filled with a fused silica filler treated by a coupling agent is applied being subjected to a hydrolysis reaction in the encapsulating material due to effects of ambient temperature and humidity, resulting in occurrence of voids during molding for encapsulation, and reduced reliability of the semiconductor device (reduced adhesive force between the encapsulating material and a silicon substrate, and reduced insulation properties between wirings).
The method of treating a filler by a coupling agent, which has a precondition of hydrolysis reaction with hydroxyl groups on the surface of a filler, has not been capable of being applied to alumina, aluminum nitride, magnesium oxide, silicon nitride, boron nitride and the like which are used as electrically insulating fillers having high thermal conductivities. Accordingly, these fillers could not enjoy a benefit of enhanced properties of the filler by improvement of wettability to a resin component.
In the presence of voids or cracks in a formed layer of encapsulating material, peeling of the encapsulating material layer from a silicon substrate further progresses originating in the voids or cracks due to shock during cutting of a wafer into individual elements.
In addition, in encapsulation of a wafer, it is critical to avoid occurrence of unfilled portions where an encapsulating material does not spread. A wafer having unfilled portions is a reject, resulting in waste. In particular, for wafers of a diameter of 6 to 8 inches (about 15 to 20 centimeters) or more, since formation of unfilled portions leads to waste of a large number of elements at the same time, it is essential to thoroughly cover the wafer so that unfilled portions are not formed.
An encapsulated semiconductor device mounted on a substrate is subjected to a temperature cycle due to on-off operation of the device and change of external temperature after mounting. During that cycle, the substrate for mounting is expanded and shrunk, and the resultant stresses are applied to the semiconductor device. The stress is concentrated particularly in the vicinity of roots of copper bumps in the semiconductor device (joints to the silicon substrate), resulting in failure of bumps, breakage of elements or the like.
For the manufacture of an encapsulating material, melting and kneading of a resin component, and subsequent solidification and grinding are essential, and it is inevitable that metal is incorporated into the encapsulating material from items of equipment used during these processes (a kneader, a crusher, etc.). In prior wire bonding used for bonding of a semiconductor functional element and a lead frame, even when a metal piece having a length of, for example, 34 micrometers is included in an encapsulating material, the included metal piece did not directly lead to a rejected element with respect to a pitch between wirings and a pitch between leads. In a chip size package, however, since a distance between wirings on the surface of an element is 25 to 10 micrometers, a possibility of metal or conductor powder of a length of 34 micrometers or more causing a short-circuit between wirings is drastically increased when such metal or conductor powder is included in an encapsulating material. Therefore, with respect to inclusion of metal or conductor powder in an encapsulating material, it is necessary to strictly control the quantity and the size thereof. A method of isolating a metal in a resin and a method of analysis thereof, which are useful for such control, have not been established to date.